UPDATE: Box Originally Attached to Experimental Craft is Missing

UPDATE: Sources are saying a box or basket of some sort was initially attached to the craft and that Falcon Heene may have climbed into it. The search continues for both boy and box.

The latest from CNN:

Officials are trying to rescue a 6-year-old boy who climbed into a balloon-like experimental aircraft built by his parents and floated into the sky over eastern Colorado.

Margie Martinez of the Weld County Sheriff's Office said a sibling saw the boy climb into the basket before the balloon took off. Since the door on the balloon was unlocked, Martinez said it's possible the boy had fallen out.

The balloon appeared to be a saucer-shaped, Mylar-coated helium balloon, not unlike a party balloon. The craft was drifting eastward, authorities said.

The helium balloon was tethered to the boy's family home in Fort Collins, the Larimer County Sheriff's Department said. The boy got into the craft Thursday morning and undid the rope anchoring it.

The aluminum-covered, dome-shaped balloon is 20 feet long and 5 feet high, the Larimer County Sheriff's Office said.

"The structure at the bottom of the balloon that the boy is in is made of extremely thin plywood and won't withstand any kind of a crash at all," said Erik Nilsson, Larimer County Emergency Manager, according to CNN affiliate KMGH.

The Federal Aviation Administration is trying to track the aircraft on radar and has notified the Denver International Airport. Shortly after noon (2 p.m. ET), the balloon was sighted two miles south of Evans.

CNN meteorologist Chad Myers estimated the wind would keep the vessel moving at 30 mph. Authorities say the craft is about 7,000 feet above the ground.

Kathy Davis, spokeswoman for the sheriff's department, said a dispatcher received a call Thursday morning, and emergency services personnel were contacted.

Davis said a balloon company had been contacted and several media outlets offered to help track the balloon with their helicopters.